If you are tired of being your cat’s doorman, adding a cat flap to your double glazed door or window feels like the obvious answer. The tricky part is understanding what is genuinely possible without ruining your glazing or creating a draughty, leaky mess.
Cat Flaps and Double Glazing: What You Need to Know
The biggest myth is that a glazier can simply cut a round hole in your existing double glazed unit and pop a cat flap in. This is not safe, and no reputable installer will do it.
A double glazed unit is a sealed sandwich of two panes of glass with a spacer bar and gas or air gap in between. Once it is manufactured and sealed, cutting into it will break the seal, likely shatter the glass, and completely compromise its insulation and strength.
The correct method is to order a brand new double glazed unit made specifically for a cat flap. The glass is cut to size in the factory, a circular or square aperture is cut before toughening, and the unit is then sealed around that opening. Your old pane is removed and replaced with this new, purpose-made unit.
Choosing the right type of cat flap
Before any glass is ordered, you need to choose the cat flap itself. The size and shape of the flap, along with the template from the manufacturer, are used to create the aperture in the glass. Getting this in the wrong order is one of the most common and costly mistakes.
Microchip vs manual locking flaps
Microchip cat flaps read your cat’s microchip or a collar tag to let only your pets inside. This can be ideal if you have neighbourhood cats trying their luck, or live near busy streets and want extra control. They are usually battery powered and have more depth to the inner workings.
Manual locking flaps are simpler and cheaper. You control entry and exit using sliding locks or a 4-way control (in only, out only, in and out, or locked). They are perfectly adequate for many homes, particularly if you do not have a problem with visiting cats.
Flap size, tunnel depth and your cat’s comfort
Cat flaps come in different sizes, including options for larger breeds or small dogs. Measure your cat’s shoulder height and width to avoid a flap that is too tight or too low, which can put stress on joints over time.
Flap size: Check the manufacturer’s recommendations for pet weight and shoulder height.
Tunnel depth: Needs to match the thickness of your new glass unit and any frame or panel it sits in.
Future pets: Consider whether a larger flap might be sensible if you might have a bigger cat or small dog later on.
The tunnel is the part that runs through the glass thickness. For double glazing, the tunnel depth and design matter so that the flap can be sealed correctly without gaps that cause draughts or water ingress.
Glass type and safety considerations
Not all glass is the same, and regulations or safety best practice can affect what is used for your new cat flap unit. In many doors and low-level windows, toughened or safety glass will already be in place.
Your installer will usually match or upgrade the existing safety specification. In locations such as patio doors, full height glazed panels or glass close to floor level, toughened glass is standard. The cat flap aperture must be cut into the glass before it is toughened, which is another reason you cannot modify an existing pane.
If you have decorative, patterned or energy-efficient coated glass, mention this early. The replacement unit can often be made to match the look and performance, so the new panel does not stand out.
Positioning the cat flap in double glazed doors and windows
Placement has a big impact on how easy the flap is to use and how tidy the finished installation looks. Your cat should be able to step through without jumping or crouching awkwardly, and you want to avoid clashes with existing hardware.
As a guide, the bottom of the flap opening is often set roughly level with your cat’s tummy, which for most cats is about 15–20 cm above the floor. For very small or elderly cats, a slightly lower height might be kinder.
Other factors to think about include the path outside. Make sure your cat is not stepping directly into a puddle, thorny shrub or drop. If the outside ground level is lower, a small external step may be helpful. Inside, ensure the flap won’t obstruct door handles, locks or trickle vents, and that you can still operate blinds or curtains comfortably.
Measuring up and information your installer needs
Accurate information at the start will avoid delays and extra cost. If you are unsure, it is often best to have the installer take final measurements, but understanding what they need helps you prepare.
Visible glass size: Width and height of the glass area, measured inside the frame bead to bead.
Spacer bar thickness: The silver or black bar between the panes, often 20–28 mm, which sets the cavity depth.
Frame or door type: uPVC, aluminium, timber or composite, plus whether it is a sliding, French or single door.
On top of this, your installer will need the exact cat flap make and model, and ideally the manufacturer’s cutting template. Using the wrong template size is a frequent cause of problems. Even a few millimetres out can leave the flap loose, stressed, or impossible to fit.
Common pitfalls to avoid
One big trap is ordering the new glass first and then choosing the cat flap later. If the aperture does not match the flap you finally buy, the unit may be unusable and have to be remade at your cost. Always finalise the flap choice before any glass is ordered.
Another issue is assuming any round hole will do. Different brands have slightly different hole sizes and screw positions. Make sure your installer works from the correct, up-to-date template for your exact model.
Poor sealing around the frame of the cat flap can also cause draughts or leaks. A professional installer will use suitable sealants and finishing trims, ensuring the flap sits snugly in the glass and the weatherproofing of your door or window is maintained.
What to expect on installation day
The actual fitting is usually straightforward and relatively quick. The installer will start by removing the existing glazed unit from the door or window, taking care not to damage the surrounding frame or interior finishes.
The new, pre-made cat flap unit is then placed into the frame and packed and sealed in position, just like a standard replacement pane. Once it is secure and weather-tight, the cat flap itself is fitted into the pre-cut aperture and sealed neatly on both sides.
Finally, any microchip flap will be programmed to recognise your cat, and the installer should show you how the locks and settings work. Most installations are completed within a single visit, with minimal mess and disturbance.
Ready to plan your cat flap in double glazing?
Getting a cat flap installed in double glazing is perfectly achievable, as long as it is done with a new purpose-made unit and the right planning. The key is to choose your flap first, share all the relevant details with a knowledgeable installer, and avoid shortcuts that could damage your glazing.
If you are unsure where to start or want to talk through your options, the team at Hunters Glass Ltd can help you choose a suitable cat flap and make sure the new double glazed unit is specified correctly. For friendly, expert advice before you order any glass, call 03301331655 or visit our cat flap fitting service page to get started.
